Field of the Invention
The present invention relates to a method of manufacturing a nitride semiconductor device used, for example, for amplification of a radiofrequency signal.
Background Art
Unlike a MOSFET, a radiofrequency device constituted by a nitride semiconductor has a gate leak current flowing when a gate-drain voltage is applied because the device has Schottky junction for its gate. It is said that degradation of the crystallinity of the semiconductor in the vicinity of the Schottky junction causes an increase in this gate leak current. When the gate leak current flows, a voltage drop occurs at an end of a gate resistor inserted in the circuit and the gate voltage is shifted to a higher positive value, resulting in a change in characteristic. Further, there is also a possibility of the device being broken by thermal runaway. It is also said that flowing of the gate leak current causes multiplication of crystal defects in the semiconductor and further increases the gate leak current.
Conventionally, in the process of manufacturing such a nitride semiconductor device, an RF burn-in step in which radiofrequency power is applied at a high temperature after a wafer process and package assembly is added to reduce the gate leak current and improve the reliability of the device. Also, a burn-in step in which DC stress is applied at a high temperature is added to reduce the gate leak current (see, for example, Japanese Patent Laid-Open No. 2014-192352).